1. Field of the Invention
This invention is generally related to the minimization of corrosion of titanium and titanium alloys especially in the substantial absence of molecular oxygen, and, more specifically, to the use of tellurium and/or selenium containing compounds as corrosion inhibitors.
2. Description of the Prior Art
Titanium and titanium alloys are, largely due to their generally high corrosion-resistance properties, widely used in industry as construction material or linings for vessels, piping and the like.
However, unacceptably high rates of corrosion of titanium and its alloys can occur, especially in the substantial absence of molecular oxygen, at elevated temperatures when in contact with certain acid media, e.g., aqueous media containing any of the strong mineral acids, such as nitric acid, phosphoric acid, sulfuric acid, hydrohalic acids (e.g., HBr, HCl, HI and HF), and the like, or any of the strong carboxylic acids, such as oxalic acid, formic acid, acetic acid and the like. Also, aqueous media containing dissolved salts of some of the above acids can vigorously attack titanium and titanium alloys.
Various compounds have been proposed for use as anticorrosive agents for titanium. Thus, U.S. Pat. No. 3,457,103 suggests use of siliceous compounds. Also, Fe.sup.3+, Cu.sup.2+ and Pt.sup.4+, as well as ions of Au, Hg, Zn, Co, Al and Mg, have been found to decrease rates of corrosion or to passivate titanium in certain media. See I. Ya. Klinov, Corrosion and Protection of Materials Used in Industrial Equipment pp. 79-90 (Consultants Bureau 1962); Corrosion Vol. 19, No. 6, pp. 217t-221t (1963); N. G. Feige et al., Chem. Eng. Prog., Vol. 66, No. 10, pp. 53-56 (1970); J. B. Cotton, Chem. Eng. Prog., Vol. 66, No. 10, pp. 57-62 (1970); T. Koizumi et al., Corrosion and Corrosion Control, pp. 318-323 (J. Wiley & Sons 1973); Titanium Science and Technology, Vol. 4, pp. 2383-2393 (1973); L. C. Covington, Titanium Science and Technology, Vol. 4, pp. 2395-2403 (1973).
Oxyanions (SO.sub.4.sup.2-, NO.sub.3.sup.1-, CrO.sub.4.sup.2-, PO.sub.3.sup.3- and CO.sub.3.sup.2-) have been found to inhibit pitting of titanium in certain specific systems containing halide ions. See, e.g., T. Koizumi, et al., supra at pp. 2388-2392. Also, NaBr has been found to inhibit titanium corrosion in fuming nitric acid. See I. Ya. Klinov, supra at p. 87.
Alloys of Pd, Pt and other metals, notably Mo, have been used to increase resistance of titanium. However, even these alloys are not completely resistant to corrosion attack by acicic aqueous media.
The following references are of interest by way of background, but are much less pertinent than those disclosed above. Tellurium metal has been included in aluminum alloys and copperbase alloys to increase the corrosion resistance to certain media. See N. Nagashima et al., 88 Chem. Abs. 196,070g (1978) and M. Tsuneaki, et al., 68 Chem. Abs. 62,282f (1968), respectively. Water soluble tellurides have been used as corrosion inhibitors for alkaline aqueous engine coolants in contact with ferrous metals, Al, Zn, Cu and Cu alloys. British Pat. No. 961,409, as cited in 61 Chem. Abs. 7952c (1964). Pitting corrosion of stainless steels containing MnTe and CrTe inclusions has been studied. G. S. Eklund, 88 Chem. Abs. 56,4025 (1978). J. E. Antill et al., 88 Chem. Abs. 80,689m (1978) investigated the influence of Te on Cs-enhanced corrosion of stainless steel, and K. Aramaki et al., 87 Chem. Abs. 159,030c (1977) observed the effects of dipropyl telluride as organic corrosion inhbitor for Fe corrosion in perchloric acid. Selenium compounds have been used as components of anti-corrosive coatings for such metals as magnesium, gold, silver, nickel and iron. See A. A. Kudryavtsev, "The Chemistry and Technology of Selenium and Tellurium", p. 229 (Collet's Publishers, Ltd.; London--1974). Se metal has also been included in Ti-containing steels. See V. G. Yusifov, et al., 77 Chem. Abs. 104,517e (1972) (Cr-Ni-Mo-Cu-Se-Ti alloy).